The present invention relates to a sputtering target for use in forming a positive electrode thin film in all-solid-state thin-film lithium ion secondary batteries, a method of producing such a sputtering target, and a positive electrode thin film produced from such a sputtering target.
A lithium ion secondary battery is attracting attention as a secondary battery with high output and large capacity, and various types of research and development are being actively pursued. The electrodes and electrolytes configuring the lithium ion secondary battery face a host of challenges that need to be researched from the perspective of energy density, charging/discharging characteristics, manufacturing process, cost of materials and so on. Under these circumstances, an all-solid-state lithium ion battery in which liquid electrolytes, which are combustible and may cause fires due to a liquid spill, are replaced with solid electrolytes, is attracting attention.
Generally speaking, solid electrolytes have an ion conductance that is a few orders of magnitude lower in comparison to liquid electrolytes, and this is a major impediment in the practical application of an all-solid-state lithium ion battery. Today, numerous research institutions and corporations are actively developing materials centering on solid electrolytes having high ionic conductivity. In recent years, an all-solid-state thin-film lithium ion secondary battery, which resolves the drawback of having low ionic conductivity by producing the solid electrolytes in the form of thin films, has been developed and placed on the market.
An all-solid-state thin-film battery is characterized in that it is thin and can be miniaturized, does not deteriorate easily, and is free from any liquid spill. The positive electrode materials and solid electrolyte films configuring this kind of thin-film lithium ion battery are prepared via the sputtering method. The present applicant previously provided a technology related to a target configured from lithium-containing transition metal oxides for forming thin-film positive electrodes for use in a thin-film lithium ion battery (Patent Document 1).
Since the foregoing positive electrode thin film is generally thick even among the materials in the thin-film lithium ion battery and takes much time to be deposited, it is desired to be formed via high speed deposition. Nevertheless, since a conventional target material that was used for forming positive electrode thin films had a high resistivity (1 to 500 kΩ or higher), it was difficult to perform DC sputtering which enables high speed deposition, and RF sputtering or the like is normally performed.